The long term goal of these studies is to understand the central and peripheral neural mechanisms underlying normal and disordered human postural control in order to gain insights into neurological dysfunctions critical to postural stability in the elderly. These studies will focus on the physiological mechanisms underlying motor coordination of automatic postural movements and the effects of well-defined central basal ganglia, cerebellar, and peripheral vestibular deficits, common in the elderly on postural coordination. Patterns of coordinated leg, trunk, and head movements will be quantified with surface forces, joint kinematics, and EMG activation patterns in subjects responding to controlled displacements of a moveable force platform. The specific aims of the proposed study are: 1. To characterize the central and peripheral mechanisms underlying: (a) the selection and (b) the scaling of NORMAL POSTURAL MOVEMENT patterns. four experiments are designed to determine the effects of surface configuration, displacement amplitude and derivatives, and displacement direction on the coordination of postural response patterns. Expected and unexpected changes in sensory and mechanical conditions will be compared to determine the roles of peripheral sensory and central predictive control on the postural response pattern and magnitude. 2. To determine the effects of CENTRAL BASAL GANGLIA and CEREBELLAR disorders on postural coordination. The coordination of automatic stepping for postural correction and the scaling of postural response magnitude will be characterized in unstable patients witH PARKINSON'S DISEASE and in patients with ANTERIOR LOBE SYNDROME to determine the unique postural deficits associated with each motor system. 3. To investigate the effects of bilateral and asymmetrical PERIPHERAL VESTIBULAR function on postural motor coordination. The ability to accurately code postural response direction and magnitude will be tested in three types of peripheral vestibular dysfunction:L bilateral loss, unilateral loss, and single semicircular canal denervation. These studies will identify critical physiological mechanisms responsible for normal and disordered human postural control so that scientifically sound and effective approaches to the assessment and rehabilitation of postural instability in the elderly may be developed.